1. Field of the Invention
The present invention relates to fluidized catalytic cracking of hydrocarbon charge stocks. More particularly, it relates to fluidized catalytic cracking wherein a substantial portion of the hydrocarbon charge is converted to hydrogen and wherein said hydrogen is recovered substantially free of contaminant gases, particularly nitrogen.
2. Discussion of Prior Art
Fluidized catalytic cracking of hydrocarbon liquids ranging from naphtha through residuum stocks is well known. In such processes, a portion of the hydrocarbon charge is converted to hydrogen and low molecular weight hydrocarbons such as methane, ethylene and ethane. Although the weight percent of charge converted to hydrogen is small (in the range of 0.5-2 wt. percent), the volume of hydrogen is considerable, considering the low molecular weight of hydrogen and the total weight of hydrocarbon charge to a fluidized catalytic cracking unit. That is, the hydrogen yield from a fluidized catalytic cracking unit charging 100,000 B/D hydrocarbon is in the range of 1.5 to 10 million SCF/D hydrogen. This hydrogen is, however, contaminated with other light gases, such as C.sub.1 -C.sub.2 hydrocarbons, nitrogen, carbon monoxide, carbon dioxide and hydrogen sulfide. Also, the hydrogen is recovered at a relatively low pressure. Heretofore recovery of the hydrogen as a product has not been economical, but with increased energy costs, such hydrogen recovery is more attractive.
In copending application Ser. No. 918,989, filed June 26, 1978 (now U.S. Pat. No. 4,206,038), a method for stripping hot, regenerated catalyst with an acid gas, such as CO.sub.2 and/or H.sub.2 S is disclosed wherein nitrogen and carbon monoxide contaminants are prevented from entering a fluidized catalytic cracking reaction zone, such that subsequently, hydrogen product may be recovered free of such non-condensible gases. Consequently, hydrogen of commercial purity may be recovered from a fluidized catalytic cracking unit.
Cracking hydrocarbon charge stocks in Fluidized Catalytic Cracking units in the presence of cracking catalysts contaminated with metals such as iron, nickel and vanadium for production of hydrogen-rich gas streams is known. For example, see U.S. Pat. Nos. 2,998,380 and 3,050,457. In such processes, low quality charge stocks such as residuum containing compounds of nickel, iron, and vanadium are cracked at low or high severities for producing gas streams rich in hydrogen and for producing cycle oils free of metal contaminants.
In many fluidized catalytic cracking processes, particularly those charging low quality charge stocks, such as metals contaminated residuum, increased hydrogen yields are desirable. Particularly in situations where such hydrogen may be employed in other refining processes.